Fluid and Electrolyte Balance

Fluid and Electrolyte Balance


imageBefore reading the chapter, say each of these terms out loud. This will help you avoid stumbling over them as you read.



[ana- up, -ion to go (ion)]

blood colloid osmotic pressure (BCOP)

(KOL-oyd os-MOT-ik)

[coll- glue, -oid like, osmo- push, -ic relating to]

blood hydrostatic pressure (BHP)


[hydro- water, -stat- standing, -ic relating to]



[cat- down, -ion to go (ion)]

colloid osmotic pressure

(KOL-oyd os-MOT-ik)

[coll- glue, -oid like, osmo- push, -ic relating to]



[de- remove, -hydro water, -ation process]



[dis- apart, -socia –unite, -ate action]



[electro- electricity, -lyt- loosening]

extracellular fluid compartment


[extra- outside, -cell- storeroom, -ular relating to]

extracellular fluid (ECF)


[extra- outside, -cell- storeroom, -ular relating to]

fluid and electrolyte balance


[electro- electricity, -lyt- loosening]

fluid compartment



[inter- between, -stit- stand, -al relating to]

interstitial fluid colloid osmotic pressure (IFCOP)

(in-ter-STISH-al KOL-oyd os-MAH-tik)

[inter- between, -stit- stand, -al relating to, coll- glue, -oid like, osmo- push, -ic relating to]

interstitial fluid hydrostatic pressure (IFHP)

(inter-STISH-al hye-droh-STAT-ik)

[inter- between, -stit- stand, -al relating to, hydro- water, -stat- stand, -ic relating to]

intracellular fluid compartment


[intra- occurring within, -cell- storeroom, -ular relating to]

intracellular fluid (ICF)

(in-trah-SELL-yoo-lar FLOO-id)

[intra- occurring within, -cell- storeroom, -ular relating to]



[ion to go]

milliequivalent (mEq)


[milli- 1/1000 part, -equi- equal, -val- strength, -ent state]



[non- not, -electro- electricity, -lyt-loosening]



[osmo- push, -recept- receive, -or agent]



[par- beside, -enter- intestine, -al relating to]

subfornical organ (SFO)


[sub- under, -fornic- arch, -al relating to]

thirst center




[dia- through, -ure- urine, -ic relating to]



[edema swelling]



[hyper- excessive, -kali- potassium, -emia blood condition]



[hyper- excessive, -natri- sodium, -emia blood condition]



[hyper- excessive, -vol- volume, -emia blood condition]



[hypo- under or below, -chlor- green (chlorine), -emia blood condition]



[hypo- under or below, -kal- potassium, -emia blood condition]



[hypo- under or below, -natri- sodium, -emia blood condition]



[hypo- under or below, -vol- volume, -emia blood condition]

intravenous injection

(in-trah-VEE-nus in-JEK-shun)

[intra- within, -ven- vein, -ous relating to, in- in, -ject throw, -tion process]

parenteral therapy


[par- beside, -enter- intestine, -al relating to]

pitting edema


subcutaneous injection

(sub-kyoo-TAY-nee-us in-JEK-shun)

[sub- under, cut- skin, -aneous relating to, in- in, -ject- throw, -tion process]



[turg- swollen, -or condition]

The phrase fluid and electrolyte balance implies homeostasis, or constancy, of body fluid and electrolyte levels. It means that both the amount and distribution of body fluids and electrolytes are normal and constant. For homeostasis to be maintained, body “input” of water and electrolytes must be balanced by “output.” If water and electrolytes enter the body in excess of requirements, they must be selectively eliminated, and if excess losses occur, prompt replacement is critical. The volume of fluid and the electrolyte concentrations inside the cells, in the interstitial spaces, and in the blood vessels all remain relatively constant when a condition of homeostasis exists. Fluid and electrolyte imbalance, then, means that both the total volume of water and the level of electrolytes in the body or the amounts in one or more of its fluid compartments have increased or decreased beyond normal limits.


Several of the basic physical properties of matter discussed in Chapter 2 help explain the mechanisms of fluid and electrolyte balance. The concept of chemical bonding is a good example. The type of chemical bonds between molecules of certain chemical compounds, such as sodium chloride (NaCl), permits breakup, or dissociation, into separate particles (Na+ and Cl). Recall that such compounds are known as electrolytes. The dissociated particles of an electrolyte are called ions and carry an electrical charge. Organic substances such as glucose, however, have a type of bond that does not permit the compound to break up, or dissociate, in solution. Such compounds are known as nonelectrolytes.

Many electrolytes and their dissociated ions are of critical importance in fluid balance. Fluid balance and electrolyte balance are so interdependent that if one deviates from normal, so does the other. A discussion of one therefore necessitates a discussion of the other.


Normal values for total body water expressed as a percentage of total body weight will vary between 45% and 75%. Differences occur because of age, fat content of the body, and gender. In newborn infants, total body water represents about 75% of body weight. This percentage then decreases rapidly during the first 10 years of life. At adolescence, adult values are reached and gender differences, which account for about a 10% variation in body fluid volumes between the sexes, appear. In young, nonobese adults, males weighing 70 kg (154 pounds) will have on average about 60% of their body weight as water (nearly 40 liters) and females about 50% (Table 32-1). Adipose, or fat, tissue contains the least amount of water of any tissue (including bone) in the body. Therefore, regardless of age, obese individuals, with their high body fat content, have less body water per kilogram of weight than slender people do. In aged individuals of either sex, body water content may decrease to 45% of total body weight. One reason for this is that old age is often accompanied by a decrease in muscle mass (65% water) and an increase in fat (20% water). In addition, with advancing age the kidneys are less able to produce concentrated urine, and sodium-conserving responses become less effective.


Functionally, the total body water can be subdivided into two major fluid compartments called the extracellular and the intracellular fluid compartments. Extracellular fluid (ECF) consists mainly of the plasma found in the blood vessels and the interstitial fluid that surrounds the cells (Table 32-2). In addition, the lymph and so-called transcellular fluid—such as cerebrospinal fluid, joint fluids, and humors of the eye—are also considered extracellular fluid. The distribution of body water by compartment is shown in Figure 32-1. Intracellular fluid (ICF) refers to the water inside the cells.

Extracellular fluid makes up the internal environment of the body. It therefore serves the dual vital functions of providing a relatively constant environment for cells and transporting substances to and from them. Intracellular fluid, on the other hand, because it is a solvent, functions to facilitate intracellular chemical reactions that maintain life. When compared according to volume, intracellular fluid is the largest (25 L), plasma the smallest (3 L), and interstitial fluid in between (12 L). Figure 32-2 illustrates the typical normal fluid volumes in a young adult male, and Table 32-1 lists volumes of the body fluid compartments for both sexes as a percentage of body weight.


We have defined an electrolyte as a compound that will break up or dissociate into charged particles called ions when placed in solution. Sodium chloride, when dissolved in water, provides a positively charged sodium ion (Na+) and a negatively charged chloride ion (Cl).

If two electrodes charged with a weak current are placed in an electrolyte solution, the ions will move, or migrate, in opposite directions according to their charge. Positive ions such as Na+ will be attracted to the negative electrode (cathode) and are called cations. Negative ions such as Cl will migrate to the positive electrode (anode) and are called anions. Various anions and cations serve critical nutrient or regulatory roles in the body. Important cations include sodium (Na+), calcium (Ca++), potassium (K+), and magnesium (Mg++). Important anions include chloride (Cl), bicarbonate (HCO3), phosphate (HPO4=), and many proteins.

The importance of electrolytes in controlling the movement of water between the body fluid compartments is discussed in this chapter. Their role in maintaining acid-base balance is examined in Chapter 33.

Extracellular vs. Intracellular Fluids

Compared chemically, plasma and interstitial fluid (the two extracellular fluids) are almost identical. Intracellular fluid, on the other hand, shows striking differences as compared with either of the two extracellular fluids. Let us examine first the chemical structure of plasma and interstitial fluid as shown in Figure 32-3 and Table 32-3.

Perhaps the first difference between the two extracellular fluids that you notice (see Figures 32-3 and 32-4) is that blood plasma contains a slightly larger total of electrolytes (ions) than interstitial fluids do. If you compare the two fluids, ion for ion, you will discover the most important difference between blood plasma and interstitial fluid. Look at the anions (negative ions) in these two extracellular fluids. Note that blood contains an appreciable amount of protein anions. Interstitial fluid, in contrast, contains hardly any protein anions. This is the only functionally important difference between blood and interstitial fluid. It exists because the normal capillary membrane is practically impermeable to proteins. Hence, almost all protein anions remain behind in the blood instead of filtering out into the interstitial fluid. Because proteins remain in the blood, certain other differences also exist between blood and interstitial fluid—notably, blood contains more sodium ions and fewer chloride ions than interstitial fluid does.

Extracellular fluids and intracellular fluid are more unlike than alike chemically. Chemical difference predominates between the extracellular and intracellular fluids. Chemical similarity predominates between the two extracellular fluids. Study Figures 32-3 and 32-4 and make some generalizations about the main chemical differences between the extracellular and intracellular fluids. For example: What is the most abundant cation in extracellular fluids? In intracellular fluid? What is the most abundant anion in extracellular fluids? In intracellular fluid? What about the relative concentrations of protein anions in extracellular fluids and intracellular fluid?

The reason we call attention to the chemical structure of the three body fluids is that here, as elsewhere, structure determines function. In this instance the chemical structure of the three fluids helps control water and electrolyte movement between them. Or, phrased differently, the chemical structure of body fluids, if normal, functions to maintain homeostasis of fluid distribution and, if abnormal, results in fluid imbalance. Hypervolemia (excess blood volume) is a case in point. Edema (discussed in detail on p. 1015), too, frequently stems from changes in the chemical structure of body fluids. Box 32-1 discusses therapies involving administration of fluids and electrolytes.

Box 32-1

Fluid and Electrolyte Therapy

The term parenteral therapy is used to describe the administration of nutrients, special fluids, and/or electrolytes by injection. The term implies that whatever is administered enters the body by injection and not through the alimentary canal. Examples of parenteral routes include intravenous injection (into veins) and subcutaneous injection (under the skin). Significant quantities of nutrient or electrolyte solutions that are injected subcutaneously must be isotonic with plasma or cellular damage will occur. Such solutions may be administered intravenously, however, regardless of tonicity, if correct rates of administration are used. The intravenous route is generally the preferred route for all fluid and electrolyte solutions. It permits the body to adjust its fluid compartments in the same way that it does after the ordinary intake of water and food. The ideal route for the absorption of nutrients and fluids is, of course, the digestive tract. However, if for any reason the digestive tract route cannot be used, parenteral administration of these substances is required to sustain life.

Parenteral solutions are generally given to accomplish one or more of three primary objectives:

Although many different types and combinations of nutrients and electrolytes in solution are available to meet almost every medical need, 85% to 95% of all individuals needing fluid therapy are treated with one or more of the seven basic solutions listed below:

Carbohydrate and water solutions not only supply water for body needs but also provide calories required for energy. Dextrose (glucose) and fructose (levulose) are the common parenteral carbohydrates. Perhaps the most frequently used parenteral solution is 5% dextrose in water (D5W).

Various carbohydrate and saline solutions are also available for parenteral use. Such solutions are of primary value in individuals who have a chloride deficit and ongoing fluid and caloric needs. Patients who are vomiting or undergoing gastric suction that results in the loss of chloride in hydrochloric acid need these solutions. Prolonged and heavy sweating and diarrhea also produce chloride deficits.

Current trends in parenteral therapy have reduced the frequency of normal saline use administered independently of other electrolytes or carbohydrates. In recent decades, normal saline as a general purpose electrolyte has been replaced by the use of Ringer’s solution, which provides more of the essential electrolytes in physiological proportions. Ringer’s solution is often described as normal saline modified by the addition of calcium and potassium in amounts approximating those found in plasma. Normal saline is still useful and widely used in cases where chloride loss is equal to or greater than the loss of sodium, however.

Potassium, lactate, and ammonium chloride solutions are specialty fluids used in the treatment of such conditions as acid-base imbalance or, in the case of potassium, administered during the healing phase of severe burns or in patients with actual potassium deficiency. In acidosis, lactate is rapidly converted by the liver to bicarbonate ions, and administration of ammonium chloride is useful in treating alkalosis. Acid-base imbalances and their treatment are discussed in Chapter 33.

Before discussing mechanisms that control water and electrolyte movement between blood, interstitial fluid, and intracellular fluid, it is important to understand the units used for measuring electrolytes.

Measuring Electrolyte Reactivity

After the important electrolytes and their constituent ions in the body fluid compartments had been established, physiologists needed to measure changes in their levels to understand the mechanisms of fluid balance. To have meaning, measurement units used to report electrolyte levels must be related to actual physiological activity. In the past, only the weight of an electrolyte in a given amount of solution—its concentration—was measured. The number of milligrams per 100 ml of solution (mg%) was one of the most frequently used units of measurement. However, simply reporting the concentration of an important electrolyte such as sodium or calcium in milligrams per 100 ml of blood (mg%) gives no direct information about its chemical combining power or physiological activity in body fluids. The importance of valence and electrovalent or ionic bonding in chemical reactions was discussed in Chapter 2. The reactivity or combining power of an electrolyte depends not just on the number of molecular particles present but also on the total number of ionic charges (valence). Univalent ions such as sodium (Na+) carry only a single charge, but the divalent calcium ion (Ca++) carries two units of electrical charge.

The need for a unit of measurement more related to activity has resulted in increasing use of a more meaningful measurement yardstick—the milliequivalent (mEq). Milliequivalents measure the number of ionic charges or electrovalent bonds in a solution and therefore serve as an accurate measure of the chemical (physiological) combining power, or reactivity, of a particular electrolyte solution. The number of milliequivalents of an ion in a liter of solution (mEq/L) can be calculated from its weight in 100 ml (mg%) by using a convenient conversion formula.

Conversion of milligrams per 100 ml (mg%) to milliequivalents per liter (mEq/L):

mEq/L=mg/100ml×10×ValenceAtomic weight


Example: Convert 15.6 mg% K+ to mEq/L

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May 25, 2016 | Posted by in ANATOMY | Comments Off on Fluid and Electrolyte Balance
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